Now while participating in a press event in London for Guardians of the Galaxy, Marvel Studios president and producer Kevin Feige confirms they are still in talks with Vin Diesel for a larger role (via Wired).

Again, we wanted an actor, someone who could actually do that emotion. During the casting process, Vin had come in for a general meeting, and he has a gigantic fanbase, in particular a Facebook fanbase. He started posting just about his meetings with us and, I think we were doing a press visit on Winter Soldier, and people were going "have you met with Vin Diesel yet?" or "How did your meeting with Vin Diesel go?" Coincidentally, it ended up being that day that we met with him, and we were talking -- and still are -- about what we could do in the future as a bigger part. But there was this odd fever in the online community about what Vin was going to do with Marvel.

Previously, Vin Diesel seemed to provide a hint what that bigger part could be as he mentioned the "merging of brands" for Marvel Studios' Phase 3 plans.